Influence of the preparative route on the properties of WOx-ZrO2 catalysts:: A detailed structural, spectroscopic, and catalytic study

Two series of tungstated zirconia (WZ) solid acids covering a wide range of tungsten surface densities (delta, W at/nm(2)) were prepared by nonconventional impregnation and coprecipitation routes, leading to samples with enhanced surface area (similar to 70-120 m(2)/g) on annealing at 973-1073 K. The materials were thoroughly characterized by N-2 physisorption, XRD, Raman, XPS, H-2-TPR, and DR UV-vis spectroscopy. The catalytic behavior of the Pt-promoted WZ catalysts (1 wt% Pt) was evaluated for the hydroconversion of n-hexadecane used as model feed representative of Fischer-Tropsch waxes. Both series of catalysts displayed a pronounced maximum in the reaction rate and a minimum in the selectivity to branched feed isomers (iso-C-16) at an intermediate tungsten density (delta(max)). Interestingly, we found that delta(max) shifted toward higher values for coprecipitated catalysts (delta(max,COP) = 6.8 W at/nm(2)) compared with the impregnated ones (delta(max,IMP) = 5.2 W at/nm(2)). This has been ascribed to a better inherent capacity of the coprecipitation route for dispersing tungsten species on the ZrO2 surface, as inferred from modeled XPS data. This determines that both the formation of highly interconnected amorphous WO, domains required for the generation of catalytically active Bronsted acid sites and the onset of growth of inactive three-dimensional WO3 crystallites (ascertained by XRD and Raman) occur at higher tungsten surface densities in WZ solids generated by coprecipitation than in those obtained by impregnation. Despite the observed shift in delta(max), the two most active samples within each series displayed nearly the same intrinsic activity per total W atoms, suggesting that a similar nature and size for the supported active WO, domains should be attained by both impregnation and coprecipitation routes at delta = delta(max). Moreover, the method of preparation was found to affect the optical and electronic properties of the supported WOx species. Thus, coprecipitation provides WZ solids displaying a lower valence-conduction energy gap, as well as enhanced reducibility for the polytungstate domains due to an improved electronical linkage with the zirconia support, in opposition to a more isolated character of the WOx clusters generated by impregnation. (c) 2007 Elsevier Inc. All fights reserved.